The present invention relates to thermotropic aromatic polyester(amide) fibers, more particularly to the monofilaments of such polymers, and to processes for obtaining such monofilaments.
The production from thermotropic aromatic polyester(amide) of conventional multifilament fibers formed of a large number of filaments of low elementary diameter (typically from about 20 to 30 xcexcm) or of unit monofilaments of large diameter (at least 40 xcexcm) by melt-spinning the polymer, generally followed by heat treatment referred to as post-polycondensation, is a known technique.
International Application WO 92/12018 (equivalent patents EP-B-517 870 and U.S. Pat. No. 5,427,165) describes in particular reinforcement assemblies intended to replace steel cables in tires, these assemblies being formed of monofilaments of liquid-crystal organic polymers having very high mechanical properties, in particular of aromatic polyester. To obtain these aromatic polyester monofilaments, the molten polymer is extruded at 340xc2x0 C. through the capillary of a spinneret, the diameter of which is 800 xcexcm and the temperature of which is 270xc2x0 C.; the liquid jet emerging from the spinneret is stretched in air (stretching ratio about 20), then solidified by passing into a thermal quenching zone. The as-spun monofilament thus obtained is taken from a winding device at a speed of 590 m/min, then subjected to the post-polycondensation heat treatment on the takeup reel: this post-polycondensation phase, which is particularly long for this type of polymer (several hours) in fact involves the treatment being carried out on a reel, generally in an oven, and not on a single-thread passing continuously through this oven. After heat treatment, the monofilaments, for a diameter of about 180 xcexcm, have the following mechanical properties: initial modulus of 4300 cN/tex, elongation at break of 2.5% and tenacity of 130 cN/tex. Owing to the liquid-crystal nature of the initial polymer, the single-threads, already in the as-spun state, have a very high initial modulus, greater than 4000 cN/tex, the post-polycondensation heat treatment being essentially intended to increase the tenacity of the spun products.
However, one major disadvantage of the as-spun monofilaments described above is that they have the special characteristic of contracting in the hot state. This property, which is probably linked to release from constraints xe2x80x9cfrozenxe2x80x9d during spinning, makes it difficult to perform the subsequent post-polycondensation phase, and it is detrimental to the quality of the heat-treated monofilaments which derive therefrom, as is explained hereafter.
It happens that unless the monofilaments are allowed the possibility of contracting freely during their heat treatment, on their support reel, the latter will develop very major tensions which may result in partial damaging thereof, or even in self-breaking. Furthermore, there is the risk of interfilament xe2x80x9cmarriagexe2x80x9d between adjacent or superposed turns, this risk being due to the combined action of the tension of contraction and of the temperature; such marriage, if it takes place, may prevent satisfactory later unwinding of the treated monofilaments.
In order to limit the above risks, although it has been attempted, before the treatment of the as-spun single-threads, to wind them up again at very low speed (several tens of meters per minute) to obtain as low as possible a tension on the support reel, this operation is costly from an industrial point of view and difficult to carry out when large lengths of monofilaments have to be treated. Attempts have also been made to utilise geometries of crossed overlapping winding limiting the contact between the threads, but the contraction then induces flexion-compression damage at the contact points.
To allow the monofilaments, on the contrary, the possibility of contracting freely during their treatment, experiments have been made using specific flexible reels which contract to a greater or lesser extent under the effect of the tension (variable diameter), this avoiding prior rewinding operations under very low tension. The use of such reels, although admittedly rather impractical and more costly, in particular reveals another major drawback of these as-spun monofilaments: their self-compression upon the thermal contraction in the majority of cases involves irreversible structural damage, revealed on the treated products by the presence of compression defects well-known under the name of xe2x80x9ckink-bandsxe2x80x9d, once a critical compression threshold, which is relatively low for this type of polymer, is exceeded.
Thus, whatever the solution adopted, none has hitherto proved completely satisfactory with respect to the various problems posed by as-spun single-threads which contract in the hot state, in particular during their thermal post-polycondensation treatment.
Some of the disadvantages described previously are moreover not specific to monofilaments of high diameter, and have already been described for conventional multifilament fibers of thermotropic aromatic polyester(amide).
Nevertheless, all these drawbacks are generally exacerbated on monofilaments owing to their greater diameter: the damage to a filament after heat treatment may for example pass unnoticed on a multifilament fiber formed of several hundreds of filaments, whereas it most frequently becomes crippling on a unit monofilament of large diameter.
The first object of the invention is to overcome the above drawbacks by proposing a new monofilament of thermotropic aromatic polyester(amide) which, in the as-spun stage, has the characteristic of not contracting when hot.
This as-spun monofilament satisfies the following conditions: Dxe2x89xa740; Te greater than 45; xcex94Lxe2x89xa70, D being its diameter (in xcexcm) or its thickness in the case of an oblong or flattened shape, Te its tenacity (in cN/tex) and xcex94L its variation in length (in %) after 2 minutes at 235xc2x15xc2x0 C. at an initial tension of 0.2 cN/tex.
When the monofilament of the invention is intended to reinforce articles of plastics material and/or of rubber, in particular tires, D preferably lies within a range from 80 to 230 xcexcm, more preferably from 100 to 200 xcexcm.
Compared with the monofilaments of thermotropic aromatic polyester(amide) of the prior art, in particular within the range of 80 to 230 xcexcm above, the as-spun monofilament of the invention has the advantage of having, for a given polymer and a given diameter D, a lower tensile modulus combined with an elongation at break which is generally higher, which constitutes an advantageous compromise. For it is known that generally, for fibers of liquid-crystal origin having very high mechanical properties, such a combination favours better flexion-compression properties, which are particularly desired when articles of plastics material and/or of rubber, in particular tires, have to be reinforced; this better compromise observed on the as-spun monofilaments is retained on the heat-treated monofilaments which derive therefrom.
Thus, preferably, the as-spun monofilament of the invention satisfies the conditions:
Mi less than 4000; Ar greater than 2, Mi being its initial modulus (in cN/tex) and Ar its elongation at break (in %).
The monofilament of the invention is obtained by means of a novel, specific spinning process which constitutes another subject of the invention, this process being characterized in that it comprises the following stages:
a) melting the polymer;
b) extruding the molten polymer through a spinneret comprising at least one extrusion capillary;
c) on emerging from the capillary, structuring the flow of the polymer by stretching in a layer of gaseous fluid, preferably air, for a predetermined time ts;
d) at the end of the time ts, thermally quenching the flow of polymer thus structured by passing it through a quenching liquid, preferably water, so as to solidify it;
e) after possibly drying it, winding the monofilament thus obtained, the time ts (in seconds) being linked to the diameter or thickness D (in xcexcm) of the as-spun monofilament by the following condition (1):
ts less than to =6xc3x9710xe2x88x926D2.
The as-spun monofilament of the invention can be used as such, or alternatively heat treated to obtain a monofilament of post-polycondensed thermotropic aromatic polyester(amide), which constitutes another subject of the invention.
The invention furthermore relates to the use of the monofilaments of the invention, be it in the state of an assembly or of a unit thread, for reinforcing articles of plastics materials and/or of rubber, and also to these articles themselves, in particular the rubber plies intended for manufacturing tires and these tires themselves.
The invention, and its advantages, will be readily understood in the light of the following description and examples of embodiment.
I-1. Optical properties of the polymers
The optical anisotropy of the polymers is tested by observing, in the molten phase (i.e. above the melting temperature of the polymer) a drop of polymer between the linear crossed polarizer and analyzer of an optical polarizing microscope (Olympus type BH2) at rest, that is to say in the absence of dynamic stress.
In known manner, if the polymer is thermotropic (i.e. liquid-crystal), the preparation above is optically anisotropic, that is to say, depolarizes light: when thus placed between a linear crossed polarizer and analyzer it transmits light (more or less colored texture); an optically isotropic preparation, under the same observation conditions, does not have the above property of depolarization, the field of the microscope remaining black.
I-2. Mechanical properties of the monofilaments
In the present description, xe2x80x9csingle-threadxe2x80x9d or xe2x80x9cmonofilamentxe2x80x9d is understood to mean a unit filament, the diameter or thickness of which (that is to say, the smallest transverse dimension of its cross-section when this is not circular), referred to as D, is at least 40 xcexcm (minimum linear density of 1.7 tex).
The above definition therefore covers equally well monofilaments of essentially cylindrical shape (i.e. with a circular section) and oblong monofilaments, flattened monofilaments or even strips or films of thickness D.
All the mechanical properties below are measured on monofilaments which have undergone prior conditioning; xe2x80x9cprior conditioningxe2x80x9d is understood to refer to the storage of the monofilaments (after drying) before measurement, in a standard atmosphere in accordance with European Standard DIN EN 20139 (temperature of 20xc2x12xc2x0 C.; moisture content of 65xc2x12%) for at least 24 hours.
The linear density of the monofilaments is determined on at least three samples, each corresponding to a length of 50 m, by weighing this length of monofilament. The linear density is given in tex (weight in grams of 1000 m of monofilamentxe2x80x94reminder: 0.111 tex=1 denier).
The mechanical properties in extension (tenacity, initial modulus and elongation at break) are measured in known manner using a Zwick GmbH and Co (Germany) 1435-type or 1445-type tension machine. The monofilaments undergo traction over an initial length of 400 mm, at a nominal speed of 50 mm/min. All the results given are an average of 10 measurements.
The tenacity (breaking load divided by linear density) and the initial modulus are indicated in cN/tex (centinewtons per texxe2x80x94reminder: 1 cN/tex=0.11 g/den (grams per denier)). The initial modulus is defined as the gradient of the linear part of the force-elongation curve, which occurs just after a standard pretension of 0.5 cN/tex. The elongation at break is indicated as a percentage.
The diameter D of the monofilaments is determined by calculation from the linear density of the monofilaments and of their density, in accordance with the formula:
D=2xc3x97101.5[Ti/xcfx80xcfx81]0.5,
D being expressed in xcexcm, Ti being the linear density in tex, and xcfx81 being the volume mass in g/cm3 (xcfx81 is equal to about 1.4 in the present case).
In the case of a monofilament having a non-circular cross-section, that is to say one which is other than a monofilament of essentially cylindrical shape, the parameter D, when then represents the smallest dimension of the monofilament in a plane normal to the axis of the latter, is determined not by calculation but experimentally, by an optical microscope on a transverse section of this monofilament, the latter being, for example, coated in a resin beforehand to facilitate cutting.
I-3. Test of thermal variation in length
The thermal behavior of the monofilaments is analysed, after prior conditioning, using a test called the xe2x80x9ctest of thermal variation in lengthxe2x80x9d, the principle of which is well-known to the person skilled in the art in the field of textile fibers.
In this test, the thermal variation in length, xcex94L, is measured by introducing monofilaments, under an initial tension of 0.2 cN/tex, into an oven which has first been set to a temperature of 235xc2x0 C.xc2x15xc2x0 C.
In practice, a known commercial apparatus of the xe2x80x9cTestritexe2x80x9d type (model MK3, sold by Testrite) is used. The useful length of the sample (without significant effect on the measurement) is 254 mm. xcex94L is measured automatically by the apparatus, by means of mechanical sensors, and the result of the measurement is read off from a digital display, after 2 minutes at the temperature of 235xc2x0 C.xc2x15xc2x0 C.; a positive variation xcex94L corresponds to dilation of the monofilaments, whereas a negative variation xcex94L corresponds to contraction of the latter.
II-1. Polymer
The initial polymer is any thermotropic aromatic polyester or polyesteramide which can be spun in the molten state. Such polyesters or polyesteramides, which are referred to as xe2x80x9cfully aromaticxe2x80x9d, are known to the person skilled in the art and have been described in a very large number of documents.
By way of example, the following patents or patent applications will be cited: EP 091 253; EP 205 346; EP 267 984; EP 508 786; EP 737 707; U.S. Pat. Nos. 3,491,180; 4,083,829, 4,161,470; 4,183,895; 4,447,592; 4,734,240; 4,746,694; 5,049,295; 5,110,896; 5,250,654; 5,296,542; JP 1992/333 616; JP 1996/260 242.
The invention is preferably implemented starting with a specific thermotropic aromatic polyester; this polymer consists essentially of recurrent units (A) of 6-oxy-2-naphthoyl and (B) of 4-oxybenzoyl: 
the molar ratio A:B lying within a range from 10:90 to 90:10, preferably 20:80 to 30:70.
Such a polymer, which is sold in particular by the company Hoechst Celanese under the name Vectra, was described in U.S. Pat. No. 4,161,470, and may be obtained by copolymerisation of p-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid, these two acids possibly being substituted. It has, in known manner, an excellent compromise of properties in terms of heat resistance, chemical resistance, ease of working and suitability for spinning, owing in particular to a relatively low melting point (referred to hereafter as Tm). A polymer of this typexe2x80x94Vectra type 900 or 950 with a molar ratio of A:B of 27:73xe2x80x94is widely known for conventional multifilament fibers (see, for example, J. Text. Inst. 1990, 81, No. 4, pp. 561-574) and has also been used to obtain monofilaments of the prior art described in Application WO 92/12018 referred to above.
II-2. Spinning
The initial polymer, for example in the form of granules or powder, is dried in a vacuum and then introduced into an extruder having one or more different heating zones. Just as for the spinning of conventional multifilament fibers, the temperatures and the dwell times imposed within these different zones are such that they permit complete melting of the polymer, stable rotation conditions and extrusion screw torque conditions affording regular supplying of the spinning pump, and finally make it possible to avoid degradation of the polymer in the extruder.
On emerging from the extruder, the molten polymer, which is then at the temperature Tx (extruder exit temperature), is transferred to a spinning pump which supplies a spinneret preceded by a filter.
The spinneret may comprise a single extrusion capillary or several, depending on whether it is desired to spin one single-thread or several single-threads in parallel; the case of a spinneret comprising a single capillary will be considered hereafter.
The diameter of the capillary, referred to as xe2x80x9cdxe2x80x9d, is not a critical parameter of the process: it may vary within a wide range, for example from 200 to 1500 xcexcm, or even more, depending on the intended diameter D. As indicated previously, the invention also relates to those cases in which the monofilaments have a cross-section which is other than circular, such a form possibly being obtained, for example, by modifying the cross-section of the extrusion capillary; for such monofilaments, the parameter d then represents the smallest transverse dimension of the capillary, i.e. its smallest dimension measured in a plane normal to the direction of flow of the polymer.
Preferably, the spinneret temperature Tf is less than the temperature Tm (melting temperature of the polymer).
On emerging from the spinneret and the extrusion capillary, therefore a liquid extrudate (flow of polymer) is obtained consisting of an elementary liquid vein in the form of a still-liquid monofilament. This liquid vein of polymer is then structured, oriented by stretching (see spin-stretch factor FEF below) in a layer of gaseous fluid, for a predetermined time ts, before penetrating into a liquid thermal quenching zone.
By convention, structuring time ts is understood here to mean the total passage time for the flow of polymer in the layer of gaseous fluid, whatever the profile or gradient of stretching of the flow in this layer of gaseous fluid.
The layer of gaseous fluid is preferably air, the thickness Ag of which may vary, for example, from several centimeters to several meters, depending on the specific conditions of implementation of the invention, in particular depending on the times ts intended. xe2x80x9cThickness Ag of the layer of gaseous fluidxe2x80x9d is understood to mean the distance between the exit from the spinneret and the entrance to the liquid thermal quenching zone. Preferably, the temperature Tc of the layer of gaseous fluid is significantly less than Tf, Tc generally being close to ambient temperature (about 20xc2x0 C.).
According to the invention, the structuring time ts (in seconds) is linked to the diameter D (in xcexcm) of the as-spun monofilament by the following condition (1):
ts less than to=6xc3x9710xe2x88x926D2.
A structuring time ts lower than the critical value to above is a necessary condition to guarantee that, whatever the diameter D intended, an as-spun monofilament which does not contract in the hot state (i.e. having a variation xcex94Lxe2x89xa70 in the test of thermal variation in length) is obtained.
Preferably, the following condition (2) is satisfied:
1.5xc3x9710xe2x88x926D2 less than ts less than 6xc3x9710xe2x88x926D2.
It is in fact desirable that the structuring times ts not be too short, if it is desired to obtain monofilaments having sufficient strength to be able to be used to reinforce rubber articles such as tires.
It has been observed that the implementation of the process of the invention in accordance with condition (2) above, for the spinning of monofilaments of a diameter D of 80 to 230 xcexcm, more preferably 100 to 200 xcexcm, in particular from the polymer having recurrent units A and B previously defined, was particularly beneficial to obtaining an initial modulus Mi of between 2500 and 4000 cN/tex, more preferably at least equal to 3000 cN/tex and less than 4000 cN/tex. For such preferred conditions, when the process is used to obtain single-threads of circular section of a diameter of 100 to 200 xcexcm, furthermore, more preferably, the following conditions are used: the spinning rate (see Vf hereafter) lies within a range from 500 to 1000 m/min and the thickness of the layer of gaseous fluid (Ag) is selected to be greater than 0.50 meters and less than 2.0 meters.
At the end of the time ts, the flow of polymer thus structured and oriented penetrates into the liquid thermal quenching zone where, in contact with the liquid agent, it solidifies and thus forms a monofilament. Preferably, the liquid thermal quenching agent is water, and its temperature T1 is preferably less than ambient temperature, for example of the order of 10 to 15xc2x0 C.
For this liquid thermal quenching operation, simple means may be used, consisting, for example, of a bath containing the quenching liquid and through which the single-thread being formed circulates. The liquid quenching time is not a critical parameter, and may vary, for example, from several milliseconds to several tenths of a second, or even several seconds, depending on the specific conditions of implementation of the invention.
It is generally on emerging from the liquid thermal quenching zone that the monofilament is taken up on an entraining device, for example a takeup roller, at a given speed referred to as the spinning rate, Vf. The ratio between Vf and the rate of extrusion Ve of the solution through the spinneret defines what is called in known manner the spin-stretch factor (abbreviated FEF=Vf/Ve).
Typically, the spin-stretch factor and the spinning rate may vary within a very wide range, for example from 2 to 50 for the FEF and from 100 to 1500 m/min for Vf.
It has been noted that the mechanical properties of the monofilaments were influenced very little by the spin-stretch factor for ranges as large as those indicated above, whereas they turned out to be particularly sensitive to the structuring time ts before the liquid thermal quenching. In other words, unexpectedly, the properties obtained are essentially dependent, at a given diameter D, on the structuring time and not on the amplitude of deformation imposed during stretching.
The as-spun monofilament thus obtained is then wound at the speed Vf on to a takeup reel. It may possibly be dried before winding, for example by passing continuously over heating rollers, or alternatively be wound in the wet state and then dried on the reel, for example in ambient air or at a higher temperature in an oven, before prior conditioning for measuring its thermal and mechanical properties.
As a general rule, the initial modulus Mi and the elongation at break Ar of the monofilament of the invention can be largely modulated by the selection of the initial polymer and of the spinning conditions, the initial modulus in particular being the higher the greater the rigidity of the polymer (use for example of thermotropic polyesteramides).
Preferably, for a better flexion/compression performance, the as-spun monofilament of the invention satisfies the following conditions:
Mi less than 4000; Ar greater than 2,
Mi being its initial modulus (in cN/tex) and Ar its elongation at break (in %).
It has furthermore been observed that most frequently an elongation at break which is even higher was associated with a value xcex94Lxe2x89xa70.2; thus, more preferably, the following conditions are both satisfied:
xcex94Lxe2x89xa70.2; Arxe2x89xa72.5.
When the monofilaments according to the invention are intended to reinforce rubber articles, in particular tires, their tenacity in the as-spun state is preferably greater than 55 cN/tex, more preferably greater than 65 cN/tex; their initial modulus, in the as-spun state, is preferably between 2500 and 4000 cN/tex, more preferably at least 3000 cN/tex and less than 4000 cN/tex.
II-3. Post-polycondensation treatment
The thermal post-polycondensation treatment, after spinning, essentially makes it possible to increase the tenacity available on the monofilaments by increasing the degree of polymerization of the polymer; generally, the more pronounced the thermal treatment, the higher the tenacity obtained after treatment. Thus, monofilaments of what is called post-polycondensed thermotropic aromatic polyester(amide) are obtained, which derive directly from the as-spun single-threads described previously.
For this treatment, the reels of as-spun monofilaments are treated in ovens in known manner, at high temperature, in a vacuum or under inert gas, for example under a flow of nitrogen, generally for several hours. The conditions of this post-polycondensation treatment, which in known manner vary according to the nature of the polymer used, are similar to those used for conventional multifilament fibers. Particular treatment conditions have been described, for example, in U.S. Pat. No. 4,161,470 for these conventional fibers, and in Application WO 92/12018 referred to above for monofilaments of a diameter of 180 xcexcm; such conditions are also given in the examples of embodiment which follow.
Preferably, the monofilament of post-polycondensed thermotropic aromatic polyester(amide) deriving from the as-spun monofilaments of the invention, of diameter D of at least 40 xcexcm, satisfies the following conditions:
Mi less than 4000; Ar greater than 2; Te greater than 100, Mi being its initial modulus (in cN/tex), Ar its elongation at break (in %) and Te its tenacity (in cN/tex). More preferably, its modulus Mi is between 2500 and 4000 cN/tex, more preferably still at least equal to 3000 cN/tex and less than 4000 cN/tex; its elongation at break Ar is preferably at least 2.5.
The as-spun monofilaments of the invention, like those in the post-polycondensed state which derive therefrom, may be used in various applications, in particular for producing or reinforcing various articles, in particular articles of plastics materials and/or of rubber, for example belts, tubes or tires.
When they are used for reinforcing articles of plastics material and/or rubber, in particular in the form of cables, they preferably satisfy the following condition (D in xcexcm):
80xe2x89xa6Dxe2x89xa6230.
A diameter of at least 80 xcexcm is preferred, taking into account the costs of cabling (necessity of limiting the number of threads in the cables for a given breaking load), whereas a diameter greater than 230 xcexcm is generally to be avoided in order to restrict the flexion-compression damage (disadvantage of large diameters under a low radius of curvature). Furthermore, a diameter greater than 230 xcexcm is not very compatible with obtaining a sufficient tenacity, in particular for reinforcing tires.
More preferably still, when the single-threads of the invention are used to reinforce tires, the following condition is satisfied:
100xe2x89xa6Dxe2x89xa6200.